Where is the default text object font (Bfont)?
The font is bfont.pfb

This is a built-in font which means it  is compiled into blender. The font file is available from the blender source tree in release/datafiles

You can access and download the file, e.g., from the GIT repository at blender.org:

http://git.blender.org/gitweb/gitweb.cgi/blender.git/tree/HEAD:/release/datafiles

In the directory listing, search for the file bfont.pfb and click in the rightmost column on "raw" to download and save the file. 

Or try this direct link:

http://git.blender.org/gitweb/gitweb.cgi/blender.git/blob_plain/HEAD:/release/datafiles/bfont.pfb